Mechanical fasteners, such as rivets, are typically used to fasten work pieces and consist often of a cylindrical shaft with a head on one end. During the fastening process the fastener is placed in a hole which is punched out or drilled before. In the case of riveting, the shaft of the rivet expands, thereby holding the rivet in place.
Riveting is commonly applied for the assembly of e.g. commercial and military aircrafts. The rivets thereby carry crucial loads from one part of the aircraft to an adjoining part. Especially for the manufacturing of aeronautic components, the provision of a correct hole diameter for riveting is of great importance and the manufacturing tolerances are very tight. If the hole diameter is too big, the rivets may not be held in place properly and may unfavorably become loose during later operation. On the other hand, if the hole diameter is too small, the rivets may be difficult to install therein, or may be preloaded in a non-optimal manner. This can cause unwanted noise or even create a joint which does not provide an optimal strength. A further point is, that if the head of the rivet is not placed perfectly within the hole or its counter sink air turbulences can occur, which increase the fuel need and therefore have to be avoided.
During riveting, but also in many other fastening methods that make use of holes, the hole diameter is therefore typically checked with a measuring apparatus before installing a fastener, such as a rivet, therein. Thereby, a test mandrel (bore gauge) is inserted into the drilled or punched hole, and the lateral distance from the measuring rod to the walls of the hole is determined. This measurement is typically performed at high speed and with a high precision, i.e. in the range of μm.
However, with the known measurement or check-methods, it is not possible to characterize the edge at the surface of the hole. While it can on the one hand be desired for some applications that the holes have a sharp edge, the provision of a counter sink is also often required. It is easy to imagine that in particular the depth of a counter sink is crucial for a proper placement of a fastener into the hole. For example, if the depth of the counter sink is not correct, the end of the fastener may protrude from the surface of the work piece further than desired. For aeronautic applications, such improperly seated fasteners can cause additional fuel consumption and may even have a negative impact on the airworthiness of the airplane.